Integrated httpcapture (taking Pebble screenshots)

Hi,

After a fun little adventure coding for the Pebble I have now added a screenshot feature to httpebble. The screen capture part in "httpcapture.c" is a bit of a hack. I wrote a little about this journey here http://memention.com/blog/2013/07/20/Yak-shaving-a-Pebble.html

This commit adds a receiving part to httpebble-ios. When a screenshot is sent to the iPhone it will be shown on the Pebble image and tapping it will make it possible to save the screenshot to the camera roll.

I used 0xFFF9 as the key for a framebuffer slice. The http.c version I use can be found in pblindex https://github.com/epatel/pblindex and the extra parts in "http.c" is in "httpcapture.c" with minimal changes in http.c. 

Hope you like it :) Would love to see it in an official App Store version (so I don't need to be compiling it myself and its easier for others to use it)